As ocean warming continues to trigger widespread destruction of coral reefs, a decade-long study of remote islands in the Central Pacific suggests these biodiversity hotspots may be able to thrive despite the threats posed by an increasingly hotter planet.

With many parts of the globe in the grip of a nearly two-year coral reef bleaching event — fueled in part by El Niño-driven ocean warming — scientists and marine conservation advocates have feared many reefs could suffer irreparable damage and fade from existence in coming decades.

A new report from UC San Diego’s Scripps Institution of Oceanography provides reason for optimism by highlighting the potential for preservation efforts. In a massive project spanning 56 islands, researchers documented 450 coral reef locations from Hawaii to American Samoa, with stops in the remote Line and Phoenix islands as well as the Mariana Archipelago.

The results — published in the journal Proceedings of the Royal Society B — show that coral reefs surrounding remote islands were dramatically healthier than those in populated areas that were subject to a variety of human impacts.

“There are still coral reefs on this planet that are incredibly healthy and probably look the way they did 1,000 years ago,” said Jennifer Smith, lead author of the study and a professor at Scripps’ Center for Marine Biodiversity and Conservation.

“The scientists were practically in tears when we saw some of these reefs,” she added. “We’ve never experienced anything like it in our lives. It was an almost religious experience.”

Teeming with sharks, manta rays, jellyfish and sea turtles, these remote locations contrasted starkly with the heavily populated areas, which were encircled by coral reefs covered in murky seaweed and lacking much of the colorful algae that helps to cement a reef.

“All of these islands have reefs surrounding them,” Smith said. “It’s just what’s growing on them. On the inhabited islands, you have some corals here and there, but most of those corals are being smothered by these weedy seaweeds.”

In recent decades, coral reef ecosystems worldwide have suffered significantly from overfishing, coastal development and dumping of toxic substances into ocean waters. Scientists have predicted that up to 70 percent of coral reefs could be lost by mid-century.

Occupying less than one-tenth of 1 percent of the ocean’s sea floor, coral reefs are estimated to be home to nearly a quarter of all marine species. They also provide several human benefits, including food, tourism and flood protection for coastline developments.

The new study will hopefully catalyze coral reef preservation efforts, which some have viewed as futile, said Stephanie Wear, senior scientist for the coral reef conservation program at the Nature Conservancy.

“I think it can energize people who are responsible for managing (reefs) that what they’re doing really does matter,” she said. “There are certainly people out there who have said, ‘There’s no point; climate change is going to be the end of reefs,’ and I absolutely disagree with that.”

Still, localized efforts to safeguard coral reefs will likely only stave off the worst impacts of global warming for so long, said Nancy Knowlton, a coral reef biologist and holder of the Smithsonian Institution’s Sant Chair for Marine Science.

“The question is, how long will reefs be able to stay healthy in the context of much more frequent warming events and ocean acidification?” said Knowlton, who previously worked at Scripps and participated in the coral reef project’s initial research. “I think most reef scientists will say there are limits.

“What this study shows is that protection in a variety of different ways buys us incredibly valuable time while we figure out how to deal with the threat posed by climate change,” she added.

After El Niño conditions in 1998 that caused widespread bleaching, studies found that some coral reefs struggled to bounce back. Bleaching occurs when water temperatures rise to such a degree that corals lose a symbiotic algae that lives in their tissue and provides food.

In such cases, the bleached limestone reefs can become overgrown with seaweed, known as macroalgae and fleshy algae, that can claim real estate once occupied by the corals. People’s leaching of nitrogen-based fertilizers into oceans and overfishing of herbivore species that mow back seaweed have been linked to this process.

In healthy conditions, reef skeletons repopulate with invertebrate coral animals as well as a vibrant, multicolored crustose coralline algaecq that works as a mortar to hold the marine city together and protect it from storms.

“After a bleaching event, it really matters what happens to all those dead skeletons,” Smith said. “Do they get colonized by big seaweeds, or do they get covered by coralline algae, which are providing settlements for baby corals and providing an environment that facilitates recovery.”

She and her Scripps colleagues discovered that while the tan-colored hard coral was only slightly more abundant in uninhabited islands as opposed to populated areas, the multicolored coralline algae was nearly 40 percent higher in the remote locations.

While many studies focus solely on hard coral and large seaweeds, this analysis focused on a more “holistic approach,” Smith said. “Our initial point was that if you only look at those two things, you’re often only describing 30 percent of what’s in the community.”

Finding abundant coralline algae in remote locations builds on previous data that suggest some reefs are better poised to bounce back from adverse conditions, said Mark Eakin, coordinator of the Coral Reef Watch program from the National Oceanic and Atmospheric Administration.

“Those reefs where we’ve seen the best bounce-back from the 1998 event are those ones on the uninhabited islands,” he said. “Those reefs that are uninhabited are those that tend to have the crustose coralline algae filling the non-coral space on the reef.”

While the most recent data from the study was taken in 2009, Smith and her team of researchers continue to monitor the Central Pacific islands they visited earlier. Thousands of photographs were analyzed over several years to determine the composition of the various reefs.

Eduardo Contreras

SAN DIEGO, CA March 17, 2016 : | Dr. Jennifer Smith, associate professor, Scripps Institute of Oceanography, stands near the beach and the Scripps pier on Thursday in San Diego, California. Scripps has recently released comprehensive assessments of Pacific Ocean coral reef health spanning 10 years.

In many of the remote island locations that boasted the healthiest coral reefs, significant bleaching has occurred in the past two years amid the latest El Niño period, Smith said.

“I can’t tell you what the outcome is, because we’re still in the middle of it,” she said.

Smith plans to return to the islands again this summer. Her team’s future research could change how scientists determine the health of coral reefs, especially if coralline algae is found to significantly facilitate coral recovery after dramatic bleaching events.